This invention relates to a pulse storage memory and, more particularly, to an addressable memory and a method of and apparatus for controlling that memory. This invention further relates to a particular use of an addressable memory for the purpose of changing the repetition rate of pulse data when such data is recorded on or reproduced from a magnetic medium.
A magnetic video recorder, such as a video tape recorder (VTR) exhibits a sufficiently wide recording bandwidth such that it can be used to record audio signals with extremely high fidelity. A conventional type of VTR, when used to record an NTSC color video signal, records such a signal in parallel slant tracks, each track having a video field recorded therein. In view of the relatively low frequencies of an audio signal, there is a far greater signal storage capacity in each slant track than is needed for the audio signal. Accordingly, it is not advantageous to record an analog audio signal in place of a video signal in the slant tracks of a VTR.
If an audio signal is encoded into a digital signal, such as a PCM data signal, the resultant pulse signals can be processed without a concomittent loss in signal information. That is, the pulse signals can be transmitted or recorded with great accuracy. However, in order to exhibit the necessary high bandwidth for magnetically recording such a pulse signal, suitable magnetic recording equipment heretofore has been very expensive. A VTR of the type now available for home video recording use is far less expensive than professional-type high bandwidth magnetic recording equipment, yet such a VTR offers a satisfactory bandwidth characteristic to permit the magnetic recording of a pulse encoded audio signal.
The recording head or heads of a VTR of the aforementioned type generally is capable of recording a single channel, such as a serial pulse train. Hence, when an audio signal is encoded into pulse form, it is convenient to serialize such a pulse encoded data signal. During recording, the serialized pulse train produced by the pulse encoding device, such as an analog-to-digital converter, need not be of the same repetition rate as the pulse recording frequency. In one example, the pulse recording frequency is a function of the VTR parameters and, therefore, is related to the television synchronizing frequencies, such as the horizontal synchronizing frequency, of the video signal which normally is recorded on the VTR. In such an example, the pulse recording frequency is higher than the serialized encoded pulse repetition rate. Similarly, when a recorded pulse signal is played back from a VTR and is reconverted into an analog audio signal, the pulse reproduction rate generally is greater than the serialized pulse repetition rate which is supplied to a digital-to-analog converter. Hence, to accommodate these different pulse rates, apparatus is needed to compress the time domain of the pulse signals during recording and to expand the time domain of the pulse signals during reproduction.
One technique which heretofore has been used for time compression or expansion has required a plurality of memory devices. Pulses of a first repetition rate are serially written into a first memory by using a write clock pulse signal whose frequency is equal to the input pulse rate. When this first memory has been filled, the stored pulses are transferred to a second memory device at, for example, a read clock pulse rate which differs from the write clock pulse rate, and subsequently the pulse signals stored in the second memory device are read out to the magnetic recording transducers. If this technique is used in combination with a VTR, the capacity of each of the memory devices must be large enough to accommodate all of the pulse signals which are to be recorded in a slant track. This is necessary to avoid any interference between the incoming pulse signals, such as those produced by the analog-to-digital converter, and the outgoing pulse signals, such as those supplied to the VTR, while accommodating the desired compression or expansion of the time domain. In view of this very high memory storage capacity needed by the aforementioned technique, the cost of such memory devices is extremely high. Hence, this technique generally is economically useful only for the time-compression or -expansion of a small number of data pulses.
Another technique for changing the time domain of a pulse signal relies upon a shift register of the so-called "first-in, first-out" type wherein write and read operations can be performed on the shift register simultaneously. However, such a shift register is quite expensive such that its cost per bit renders it economically undesirable. Also, the control circuitry which must be used with such a shift register adds to the overall cost in carrying out this technique.